Hydraulic systems are used in many technical applications, for example, in agricultural and forestry machines as well as in aeroplanes and ships. Hydrostatic pumps and engines are essential components of hydraulic mechanisms. They transform mechanical energy into hydraulic energy and vice versa. Hydrostatic machines and their standard fluids based on mineral oils have developed to a high technical standard. In order to withstand the operating loads, engineers have made considerable effort to find the best combination of additives. The fluids in this case are not biologically harmless and often toxic. Fluids that are environmentally friendly cannot include additives that are toxic. Therefore, many environmentally friendly fluids have worse tribological qualities than standard fluids. If these liquids were used, important tribological qualities of the fluid must be found in the materials instead. This can be achieved by redesigned coatings. Considerations of displacement units show the most stressed tribological systems. These systems are: cylinder/valve plate, guide shoe/swash plate, plunger/cylinder bore and ball joint/guide shoe. In order to gain basic information about the quality of the coatings, experiments have to be performed using a tribometer. An experimental setup according to Siebel/Kehl is used for this. As a result of the experiments, coatings such as TiAlN/Al₂O₃ or CrAlN, which can be used for milling and drilling applications, have worse results than conventional materials such as bronze/steel. Improvements are achieved by adding carbon; however, the results are highly dependent upon the application method. Coatings that are graduated like HfCg show a low friction coefficient and wear rate. In addition to the tribological qualities of the coatings, the durability as well as the scratch resistance are important for the application. These considerations are especially important in the case of thin PVD coating which are used in applications with insufficient hydrodynamic lubrication.